On 03/03/21 14:56 +0000, Jonathan Wakely wrote:
On 01/03/21 09:56 +0100, Richard Biener via Libstdc++ wrote:
On Sun, Feb 28, 2021 at 10:53 PM Hans-Peter Nilsson <[email protected]> wrote:
On Fri, 26 Feb 2021, Thiago Macieira via Gcc-patches wrote:
On Friday, 26 February 2021 11:31:00 PST Andreas Schwab wrote:
> On Feb 26 2021, Thiago Macieira wrote:
> > On Friday, 26 February 2021 10:14:42 PST Andreas Schwab wrote:
> >> On Feb 26 2021, Thiago Macieira via Gcc-patches wrote:
> >> > - alignas(__alignof__(ptrdiff_t)) ptrdiff_t _M_a;
> >> > + alignas(__alignof__(int)) int _M_a;
> >>
> >> Futexes must be aligned to 4 bytes.
> >
> > Agreed, but doesn't this accomplish that?
>
> No. It uses whatever alignment the type already has, and is an
> elaborate no-op.
I thought so too when I read the original line. But I expected it was written
like that for a reason, especially since the same pattern appears in other
places.
I can change to "alignas(4)" (which is a GCC extension, I believe). Is that
the correct solution?
It's not a GCC extensions. You're thinking of alignas(obj) which is a
GCC extension.
IMNSHO make use of the corresponding atomic type. Then there'd
be no need for separate what's-the-right-align-curse games.
That won't work though, because we need direct access to the integer
object, not to a std::atomic<int> which contains it.
Or align as the corresponding atomic type (in case using an actual
std::atomic<int> is undesirable). OTOH the proposed code isn't
any more bogus than the previous ...
The previous code accounts for the fact that ptrdiff_t is a typedef
for an unspecified type, and that some ABIs allow struct members to have
weaker alignment than they would have otherwise.
e.g. __alignof__(long long) != alignof(long long) on x86.
Yes, I know ptrdiff_t isn't long long on x86, but since it's a typedef
for some target-specific type, it's still possible that
alignof != __alignof__. So alignas(__alignof__(T)) is not necessarily
a no-op. So not bogus.
For int, there shouldn't be any need to force the alignment. I don't
think any ABI supported by GCC allows int members to be aligned to
less than __alignof__(int). Users could break it by compiling with
#pragma pack, but I have no sympathy for such silliness.
Jakub said on IRC that m68k might have alignof(int) == 2, so we'd need
to increase that alignment to use it as a futex.
N.B. std::atomic and std::atomic_ref don't use alignas(__alignof__(T))
they do this instead:
static_assert(is_trivially_copyable_v<_Tp>);
// 1/2/4/8/16-byte types must be aligned to at least their size.
static constexpr int _S_min_alignment
= (sizeof(_Tp) & (sizeof(_Tp) - 1)) || sizeof(_Tp) > 16
? 0 : sizeof(_Tp);
public:
static constexpr size_t required_alignment
= _S_min_alignment > alignof(_Tp) ? _S_min_alignment : alignof(_Tp);
Using std::atomic_ref<T>::required_alignment would give that value.
For something being used as a futex we should just use alignas(4)
though, since that's what the kernel requires.
